Automatic shut-off lighting controls are used to save electrical energy, and are often required by legislated energy codes. Occupancy sensors are intended to keep lights ON while people are within the detection area. Detection is based on movement, which is typically sensed using passive infrared (PIR) or ultrasonic doppler shift means. When the occupant is stationary or does not move to a great enough degree to trigger detection, then the lights will turn OFF. To avoid this undesired consequence, a timer is employed to delay the OFF event. Ideally, this timer is set to a time that is at least the time between occupant movements such that the occupant will move again before the lights turn OFF. Determining the ideal duration of the timer can be a difficult chore. Occupant movements may be repetitive, but the interval between movements varies. If the timer duration is too short, then it is likely that the lights will turn OFF on the occupant. If the timer duration is too long, then the lights will remain ON longer than necessary after the occupant leaves the room, wasting energy. A false OFF can be a safety problem as well as an annoyance, so a longer than necessary time duration usually takes precedence over the need for energy savings. Occupancy sensors with manual timer settings may need to be re-adjusted until the occupant is satisfied. Sensors that have been in service for a while are often found with the time setting increased to the maximum or disabled altogether.
Manufacturers of occupancy sensors have attempted to address this problem by incorporating an automatic means to adjust the timer duration. One technique measures the interval between movements and tries to determine an ideal time-out setting based on a statistical interpretation of movement history. A reactive method increases the time-out setting in response to a false OFF event. A false OFF is assumed if movement occurs immediately after the OFF event.
There are problems with these existing methods. A statistical approach requires a great deal of data before it can zero in on the best setting, making it inherently slow to adjust in new occupancies or when the type of occupancy changes. The frustration and annoyance of occupants can be high due to false OFF occurrences while the sensor “learns the room.” Stories of adaptive timing schemes being manually disabled are legend. In contrast, the reactive approach offers faster adaptation by increasing the timer in response to a single false OFF event. However, the drawback to this approach is that the timer duration will never be reduced when the type of occupancy changes. It is also more likely that all reactive sensors will eventually arrive at the maximum timer setting, wasting energy. Currently available reactive sensors typically make large adjustments in the timer (e.g., 1.5 times the existing value) so that the sensor reaches the maximum time-out very quickly to reduce annoyance to the occupants, but at is the expense of energy savings.
Manufacturers also are concerned with false ON events since such occurrences waste energy. One proposal to reduce false ON events is to use a fault tolerant mode that requires two subsequent initial detection movements before the sensor turns ON. This method is not ideal since it introduces a time lag. For example, with the PIR detection method, the occupant must move through two optical segments before the lights turn ON. This may require further travel into a dark room, compromising safety.